WASHINGTON—U.S. companies hired at the slowest pace in more than five years in May and nearly half a million people dropped out of the labor force, clouding the outlook for Federal Reserve officials ahead of their policy meeting this month.Nonfarm payrolls rose by a seasonally adjusted 38,000 in May, the weakest performance since September 2010, the Labor Department said Friday, missing the estimate of economists surveyed by The Wall Street Journal by over 100,000.Revisions showed employers added a combined 59,000 fewer jobs in April and March than previously estimated. Together, May’s weak job growth and the revisions bring the average monthly job gains in the past three months to 116,000, a sharp slowdown from the average 219,000 growth over the prior 12 months.